Arrays of electrodes for neural stimulation are commonly used for a variety of purposes. Some examples include: U.S. Pat. No. 3,699,970 to Brindley describes an array of cortical electrodes for visual stimulation. Each electrode is attached to a separate inductive coil for signal and power. U.S. Pat. No. 4,573,481 to Bullara describes a helical electrode to be wrapped around an individual nerve fiber. U.S. Pat. No. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinal prosthesis for use with the flat retinal array described in de Juan.
It is well known that the resolution of light perception on the retina is highest at the fovea, and significantly lower at the periphery of the retina. Resolution reduces gradually across the surface of the retina moving from the fovea to the periphery.
Applicant has discovered, through experimental use of a retinal prosthesis, that a very small amount of power is needed to stimulate the perception of light near the fovea; while a much larger amount of power is needed to stimulate the perception of light further from the fovea. The resolution of a retinal electrode array is limited by the size and spacing of the individual retinal electrodes. The size of a retinal electrode is limited the amount of power that must be transferred from the electrode to neural tissue, to create the perception of light. As electrode size decreases, or power increases, charge density on the electrode increases. At high charge densities, electrodes tend to corrode, or dissolve in a saline environment. Charge density is the primary limit on how small electrodes can be made and how closely that can be placed.